1. Field of the Invention
The present invention relates to a system for drilling wellbores and more particularly to drill strings that include a bottomhole assembly that has a force application system that continuously or almost-continuously applies force on the drill bit to provide for continuous drilling and further has at least one housing or collar, which remains stationary with respect to the wellbore inside during the continuous drilling process. A set of sensors whose measurements are sensitive to the axial movement of the bottomhole assembly are integrated into the collar, which sensors take measurements while the collar is stationary while the drilling is continuing. This invention also relates to a downhole thruster system that includes an integrated steering system for drilling the wellbore along a prescribed trajectory.
2. Description of the Related Art
Wellbores are drilled in subsurface formations to recover oil and gas. Drilling is usually performed by a drilling assembly (also referred to as the xe2x80x9cbottomhole assemblyxe2x80x9d or xe2x80x9cBHAxe2x80x9d) conveyed into the wellbore by a tubing, usually a coiled tubing or a jointed pipe tubing. The BHA contains a drill bit at the bottom end of the BHA. The drill bit is rotated by a mud motor in the BHA and/or by rotating the drill pipe from the surface. For effective penetration of the drill bit into the formation, weight on bit (xe2x80x9cWOBxe2x80x9d) must be maintained within an acceptable range. Excessive WOB can cause the drill bit to become wedged in the wellbore bottom or damage the mud motor and other BHA components, while relatively small WOB can reduce the drilling rate or the rate of penetration (xe2x80x9cROPxe2x80x9d) to a level which impairs drilling effectiveness.
A thruster in the drill string (usually a part of the BHA) is sometimes used to apply force on the drill bit and to maintain and control the desired WOB. Such thrusters usually are hydraulically-operated. A thruster usually has a housing connected to the drill pipe and a mandrel or piston connected to the lower part of the BHA. The hydraulic pressure generated in the BHA is applied to the piston, which moves the piston axially (i.e. along the wellbore axis) thereby applying force and thus WOB on the drill bit during the drilling process.
There are basically two methods utilized for drilling with the hydraulic axial force generated by a thruster: The first case is when the drill pipe above the thruster can be continuously lowered, i.e., moved into the wellbore. If the axial stick slip motion of the drill pipe does not exceed the available travel distance of the piston, then the drill pipe is continuously lowered. The rate of lowering the drill pipe must, however, be the same as the rate of penetration of the drill bit into the rock formation. The second case is when the stick slip motion is such that it intermittently causes the thruster to fully extend and then collapse, then the so-called xe2x80x9cstepwisexe2x80x9d process is more appropriate. During the stepwise process each time after the piston has been fully, it shifted into the initial or the collapsed position lowering of the drill pipe. The thruster piston is continuously extended to drill the wellbore until the piston is fully extended. The drill string is then lowered by the travel distance of the piston and the process is repeated. This method can be aided by stopping and starting the pumps or at least lowering the drilling fluid flow rate and subsequently resuming the rate to the normal level. The stepwise process allows drilling under different stick slip conditions but has the disadvantage of changes of the feeding rate of the drill pipe and also, potentially, changes of the flow rate.
In order to further reduce the stick slip effects on the drilling assembly, to eliminate the reactive force on the drill pipe, and to dynamically uncouple the drill string from the BHA, the thruster can be combined with a locking device that connects the upper part of the thruster to the drill pipe. The same stepwise process for moving or lowering the drill pipe would be applied with the additional locking and unlocking of the thruster top-end or with the drill pipe positioned on top of the thruster to the borehole wall. Stopping and starting the pumps provides the additional advantage of applying only the axial force to the drill bit which is needed to axially move the drill pipe without the need to apply the incrementally larger force to create the WOB.
It is desirable to have thruster systems which can continuously apply force on the drill bit and carry out downhole measurements. International Application No. WO 99/09290 describes a drill string with a thruster system for drilling wellbores. Such a system, however, does not allow for continuous drilling of the wellbore. International Patent Application No. WO 97/08418 describes a drill string which includes two serially coupled thrusters which cooperate with each other to substantially continuously apply force on the drill bit but does not provide the desired downhole sensors. The trend in the oil drilling industry has been to incorporate a variety of sensors in the drilling assembly to take a variety of measurements-while-drilling the wellbore. Such sensors are usually referred to as measurement-while-drilling or (xe2x80x9cMWDxe2x80x9d) devices. Logging devices, such as formation resistivity sensors, acoustic sensors, etc., are sometimes referred to as the logging-while-drilling or (xe2x80x9cLWDxe2x80x9d) sensors. For the purpose of this invention, the terms MWD and LWD are used interchangeably.
It is known that some of the MWD measurements are relatively sensitive to motion, i.e., it is either preferable or necessary to make such measurements while such sensors are not moving in the wellbore. For the purpose of this invention, such measurements are referred to as the motion sensitive measurements. Additionally, it is preferable to have a continuous motion drill string that can be steered downhole so as to drill the wellbore along a preselected or desired well path. Such a steering system may be a closed loop system based on a preprogrammed well trajectory or wherein the drilling course is adjusted by sending commands from the surface. The present invention provides a drilling system wherein a thruster system continuously or near continuously applies force on the drill bit while allowing the motion sensitive sensors to make stationary measurements. The present invention further incorporates a steering device for automatically maintaining the drilling along a prescribed well path.
The present invention provides continuous or near continuous motion drill strings which include motion sensitive and other MWD sensors which take stationary measurements while the drilling assembly is continuing to drill the wellbore. For simultaneous continuous drilling and stationary measurements, the present invention provides a drilling assembly wherein a force application system almost-continuously applies force on the drill bit while maintaining a housing or drill collar section stationary. Motion sensitive sensors carried by the drill collar take stationary measurements. A steering device between the drill bit and the force application system maintains drilling of the wellbore along a prescribed well path.
To drill a wellbore, the drilling assembly of the present invention is conveyed by a tubing into the wellbore from a surface location. The drilling assembly, in one embodiment, includes two serially coupled thrusters, each having a housing that can be locked on to the wellbore and a force application member that can be moved from a first retracted position to a second extended position. The housing of the first force application device is locked in the wellbore. The force application member moves from the retracted position to the extended position applying force on the drill bit, which causes the drill bit to penetrate the formation. The force application member continues the application of the force until it is fully extended. The second force application device is then locked onto the wellbore and the first force application device unlocked from the wellbore. The second force application device applies pressure on the first force application member, causing it to move to its retracted position. After the first force application member has moved to its retracted or collapsed position, it is again locked to the borehole wall and the second force application is unlocked from the borehole. Either by continuously lowering of the drill pipe or through a stepwise lowering of the second force application member, the first force application member is then moved into its retracted position. The above process is repeated to continue the drilling process. The force applied on the drill bit by the first force application device may be constant and continuous.
In an alternative embodiment, a single continuous motion traction device is utilized to continuously apply force on the drill bit. A housing above or uphole of the continuous motion traction device remains stationary with respect to the wellbore for a predetermined travel of the traction device. In each of the drilling assemblies according to the present invention, at least one housing or drill collar remains stationary relative to the wellbore, while drilling continues. One or more motion sensitive sensors are provided on one or more of the housings of the force application system. Such sensors take measurements when the housing carrying such sensors is stationary. The present invention preferably integrates such sensors into the housings. Such sensors include a nuclear magnetic resonance sensor which is particularly susceptible to movement. The stationary housing can provide a stable platform for such sensors. Other sensors that can be integrated include a direction measuring sensor or directional sensor system, which would include at least one or more accelerometers and at least one gyroscope or a magnetometer. The combination of the measurements from the accelerometers and the gyroscopes or the magnetometers provide full directional measurement capability. Preferably three axis accelerometers are used in the directional sensor of the present invention. An acoustic sensor system may be incorporated in one of the housings. Such a sensor system would include at least one transmitter and one or more acoustic detectors spaced apart from the transmitter. Acoustic sensors provide porosity measurements and bed bound any information. A nuclear sensor may be incorporated into a housing of the present system to determine the density and the nuclear porosity of the formation surrounding the wellbore. A formation testing device usually requires extracting a fluid sample from the formation which requires the tool to remain stationary. In the present invention, a formation testing device is included in one of the housings. The above described sensors tend to be particularly sensitive to the axial movement of the sensor. However, other sensors, such as a pressure sensor may be used to determine the reservoir pressure. Stabilizers may be incorporated in the housings to reduce the vibration of the housings, thereby providing more stable platform for the motion sensitive sensors.
Thus, the present invention provides a drilling assembly that continuously exerts force on the drill bit to cause the drill bit to continuously drill the well while making selected measurements in a stationary mode. A variety of other sensors may also be incorporated into the housings and/or in other sections of the drilling assembly.
The continuous motion drilling assembly of the present invention, in one embodiment, also includes a steering device, preferably below or downhole of any thruster in the drilling assembly. Such a steering device includes one or more independently adjustable force application members or ribs. Each such member extends outward from the drilling assembly to apply selected amount of force on the wellbore wall. A control unit controls the applied force to maintain the drilling assembly along a presented or predetermined well trajectory or path.
Each embodiment of the drilling assembly of the present invention preferably includes a processor (also referred to as the xe2x80x9ccontrol unitxe2x80x9d or a xe2x80x9cprocessing unitxe2x80x9d) that includes one or more microprocessor-based circuits to process measurements made by the sensors in the drilling assembly at least in part, downhole during drilling of the wellbore. The processed signals or the computed results are transmitted to the surface by a telemetry unit in the drilling assembly. The desired downhole trajectory may be programmed into a memory of the processor. The processor then controls the force applied by the force application members to steer the drilling assembly along the prescribed well path. The processor also controls the operation of the sensors and other devices in the drilling assembly.
Examples of the more important features of the invention thus have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto.